This is the best performing of the roof types and is suited to all applications although it is the most expensive one and does not lend itself so well to remedial applications. This is because it often requires a re-roof or the original roof level needs to be raised.
Vapour Control Layer (VCL)
All non vented roofs must incorporate a VCL such as high performance Permavent Reflect. A VCL is usually positioned just behind the plaster board. BS:5250 recommends that a VCL should to be a minimum 500 gauge polythene (plastic DPM) sheeting and all corners and laps should be minimum 75mm and taped with 'Tactape'. With a warm roof it is acceptable to tape the joins of a foiled insulation with the appropriate foiled tape ensuring all joins and peripheral edges are sealed. Vapour will travel through or manifest itself in the joins of foil backed plasterboard if not detailed correctly. Given the difficult nature of vapour sealing foil backed plasterboard we do not recommend its exclusive use as a VCL. Care should be taken with cables and pipes that break the VCL.
Air Sealed Roofs
If the dwelling is to be air tested then all laps and joins should be carefully taped and peripheral edges sealed. All pipes and apertures should be sealed and an air tight loft hatch used.
Air Permeable Roof Coverings.
As vapour travels through Permavent it must then be released to the atmosphere. Most roof coverings (concrete, plain tiles etc) allow what is termed 'fortuitous ventilation' and are classed as air permeable roof coverings. This means that air can travel around them and vapour will not get trapped in the batten space between the breather membrane and tiles causing 'interstitial condensation'. Some metal sheet and man made slate roof coverings may not allow sufficient air circulation and the batten space should therefore incorporate additional ventilation such as counter battens with ridge and eaves ventilation.